Motors often have problems with exciting forces in its rotating direction (hereinafter provisionally referred to as “rotating exciting forces”). The rotating exciting forces are broadly divided into cogging torque with no current applied, and torque ripple with a current applied.
It is known that when N denotes the number of slots and P denotes the number of poles (N and P are positive integers), the order of harmonics of cogging torque is the lowest common multiple of N and P.
For example, the respective orders are compared between a motor with 8 poles and 12 slots and a motor with 10 poles and 12 slots. The lowest common multiple of 8 and 12 of the former is 24, whereas the lowest common multiple of 10 and 12 of the latter is 60. Since the order of the cogging torque per turn of the rotor of the latter motor is larger than that of the former, the peak value of the cogging torque of the latter motor is reduced.
As such, motors with (12±2)n poles and 12n teeth (hereinafter provisionally referred to as “12-slot series motors”) are recognized as promising motors with little vibration and noise. Particularly, fields where smooth torque transfer is necessary (e.g., electric power steering (EPS) for vehicles and fan drive motors) require lower rotating exciting forces. Thus, the 12-slot series motors are used in these fields (for example, Japanese Patent Application Laid-Open No. 2001-204147).
However, since the winding directions and connection of concentratedly-wound armatures in the 12-slot series motors are more complicated than those of the motors with 8 poles and 12 slots, a problem with deteriorating industrial productivity is known.
In order to address this problem, in Japanese Patent Application Laid-Open No. 2010-193675, two connection nozzles are provided per phase, by which respective armature coils are wound parallel in opposite directions. Accordingly, the armature coils can be wound with the connection nozzles operated in the same direction, thus indicating that Japanese Patent Application Laid-Open No. 2010-193675 discloses a technique for increasing the productivity.